This invention relates to a vehicle drive system as, for example, a front wheel drive system embodying a hydraulic motor for a vehicle. It has been known to provide auxiliary, hydraulic front wheel drives in vehicles as, for example, motor graders. Typically, the hydraulic motors used to drive the front wheels of the vehicle have been supplied with hydraulic fluid under pressure from the pump source of the existing hydraulic system on the vehicle which typically will be a pressure compensated pump. Prior art indicative of this approach includes U.S. Pat. No. 3,984,978 issued Oct. 12, 1976 to Alderson.
Such systems, while operative for their intended purpose, have certain disadvantages. For example, they do not utilize a flow and pressure compensated pump which may be of considerable advantage in connection with other hydraulic circuits on the vehicle utilized for driving various implements. Secondly, if one of the hydraulically driven wheels loses traction and begins to spin, the flow control of the hydraulic motor will automatically operate to destroke the motor. This results in the motor driving the wheel at a higher and higher speed with a progressively decreasing torque until such time as the torque level becomes so low that the wheel again grips the underlying terrain. In the meantime, vehicle handling qualities are reduced due to the lack of traction and the spinning of the wheel.
When it has been attempted to utilize a flow and pressure compensated pump in such systems, the difficulty of loss of traction becomes even more pronounced. In the usual case, the drive provided to the front wheel is insufficient to propel the vehicle in and of itself and is employed principally to improve the performance of the vehicle. For example, when utilized in a motor grader, the small amount of drive provided by the front wheel will be sufficient to enable the grader to grade on a fairly steep side slope without the front of the vehicle tending to slide downhill. It is also helpful in assuring good handling of the vehicle when operating in mud or snow.
In any event, because the hydraulic motors propelling the front wheels cannot propel the vehicles in and of themselves, their typical load and pressure compensating system continually sense that the motor is overloaded under normal traction conditions. That is, because the motors are lugging, they will not take the full hydraulic flow they could demand.
Consequently, the typical load signal from such motor elevates and, when directed to a flow and pressure compensated pump, the pump increases its pressure. As long as normal traction prevails, the pump output pressure spirals upwardly until maximum pressure is obtained.
At some point in the cycle, traction may be lost and the wheels will begin to spin. The hydraulic motor driving the wheel, through its flow and pressure compensating control strokes back thereby causing its speed to increase and torque to decrease until the torque becomes so low that the wheel reestablishes normal traction. The cycle will then tend to repeat itself.
As a consequence, as far as is known, it has been impossible to unite flow and pressure compensated pumps with hydraulic vehicle drive systems.